The enhanced transmission method and system proposed in the present invention, it is especially useful in 3GPP mobile networks but it could be used also in networks using any wireless transmission technology including: networks using 2G radio access technology (GSM, GPRS, EDGE etc.), 3G Technology (UMTS, HSDPA, HSUPA, etc.), 4G LTE (Long Term Evolution), as well as WIMAX.
A technique frequently used to improve the performance of, for example, 3G wireless networks is the High-Speed Downlink Packet Access HSDPA technology. HSDPA is a packet-based data service in third generation (3G) W-CDMA (Wideband Code Division Multiple Access) systems, which provides high-speed data transmission (with different download rates e.g. 7.2/10.8/16.2/21.6/28.8/43.6 Mbps over a 5 MHz bandwidth) to support multimedia services.
HSDPA comprises various versions with different data speeds and features. In table 5.1a of the release 9 version of 3GPP TS 25.306, it is shown maximum speeds of different device classes and the combination of features they support.
In order to reach yet higher peak rates (i.e. 28.8 Mbps with 3GPP Release 7 and 43.6 with Release 8) the MIMO (Multiple Input Multiple Output) feature is used in HSDPA. And it is in systems using MIMO techniques where this invention is preferably going to be applied.
In MIMO systems, both the transmitter and the receiver are equipped with multiple antennas in order to improve the system performance (capacity and user throughput performance of the networks). The basic MIMO feature as standardised in 3GPP Release 7 is based on two transmitter antennas at the node B (or an analogous network entity such as an eNode B or BTS) 11, 12 and two receiving antennas (at the UE). At the transmitter, the data can be split into one or two data streams (primary transport block 13 and optionally secondary transport block 14) and transmitted through the two antennas using the same radio resource (i.e. same transmission time interval and HSDPA codes). The primary and secondary transport block can be for example HS-DSCH channels.
If only the primary transport block (13) is transmitted, it is called a single stream MIMO transmission and if both transport blocks (13, 14) are transmitted, it is called a dual stream MIMO transmission. Generally speaking the base station scheduler can decide to transmit one or two transport blocks to a UE in one TTI in the serving cell, but in some systems or scenarios the dual stream mode is not possible e.g. when users have bad radio conditions leading low Signal to Interference plus Noise Ratios and only single stream transmission is used.
A generic downlink transmitter structure to support MIMO operation is shown in FIG. 1. The primary and optionally the secondary transport blocks are each processed 15, 16 (channel coding and interleaving), then spread and scrambled (17, 18) and subsequently weighted by precoding (PCI) weights w1, w2, w3, w4 (called MIMO precoding weights) and added (19, 20). Channel coding, interleaving and spreading are done as in non-MIMO mode. The resulting channels after MIMO precoding (i.e. MIMO channel #1 21 and MIMO channel #2 22) are mapped (23, 24) onto Primary and Secondary Common Pilot Channels (P-CPICH 25 and S-CPICH 26) respectively before being fed to the first and second physical antennas respectively. The two streams of data are recovered by the UE from the signals received via its two Rx antennas.
Thus, for the MIMO feature to work both the network and the terminals need to be MIMO-enabled. In order to deploy MIMO and transmit two parallel data streams, two power amplifiers are required per sector (one for each of the two antennas). In order not to use an entire carrier for MIMO only (5 MHz), it is more efficient and practical to use the same carrier for MIMO devices as used for non-MIMO devices (e.g. HSDPA legacy terminals).
If the Node B schedules a single transport block (single stream transmission, i.e. the secondary transport block is not transmitted) in a cell to a UE in one TTI (transmission time interval), it uses only the precoding vector (w1, w2) for transmission of that transport block through the two branches (this can be done by making the precoding weights w3=w4=0). If the Node B schedules both transport blocks in a cell to a UE in one TTI, it may use two orthogonal precoding vectors (w1, w2) and (w3, w4), to transmit the two transport blocks. The precoding vector (w1, w2) is called the primary precoding vector which is used for transmitting the primary transport block and the precoding vector (w3, w4) is called secondary precoding vector which is used for transmitting the secondary transport block, respectively.
In FIG. 1, it is supposed that both single stream and dual stream mode are allowed. If only a single stream mode is allowed in said base station, the structure will be the same but blocks corresponding to the secondary transport block (14, 16, 18 and precoding weights w3, w4) would not exist.
In an exemplary embodiment, the precoding weights w1 and w3 are constant real valued scalars and the precoding weights w2 and w4 are variable complex valued scalars. The pre-coding weights are used as part of the MIMO transmission chain defined in 3GPP (see TS 25.214) and they are selected for each specific transmission (that is, for each MIMO user equipment) by the node B. The weights selected may be signaled by the node B to the UE during the transmission. In an exemplary embodiment, the UE (user equipment) determines a preferred value of the precoding weights and send them to the node B (said information is called Precoding Weight Indication PCI), together with channel quality indication (CQI). Based on said PCI and CQI reports, the Node B decides for example: whether to schedule one or two transport blocks (i.e single stream or dual stream transmission), whether to use the preferred values sent by the UE for the precoding weights or not, what transport block size and modulation scheme to use . . . . Generally speaking, the defined weights are selected by the Node B, for example every TTI (e.g 2 ms) to optimize the transmission (for example, selecting the weights which enables the highest throughput to be transmitted for a given BLER (Blocked Error Rate) target) and they are usually selected for each MIMO user (that is, for each MIMO user equipment) taking into account information in the uplink messages of said MIMO user equipment (even said precoding weights could be selected by the MIMO user equipment as stated before).
When introducing MIMO in a system, it is indispensable to have two transmission branches (RF chains), maybe including two power amplifiers (140, 190) each one connected to the physical antenna. In order to optimise the usage of the power resource it is highly desirable to balance the power between the two power amplifiers. Whilst MIMO channels are intrinsically perfectly power balanced, all the remaining channels need to be transmitted with equal power by each power amplifier. To this end, two techniques can be used: a first one is the use of transmission diversity (using “Space Time Transmit Diversity” (STTD) for all non-MIMO channels except for the Synchronisation Channel for which “Time Switch Transmit Diversity” (TSTD) is used). Another technique is referred to as Virtual Antenna Mapping (VAM) in this description, and is discussed herein after.
Virtual Antenna Mapping is an alternative which is aimed at solving this issue fulfilling above-mentioned requirements. Hence, this technique enables power balancing of the power amplifiers whilst not impacting on the performance of legacy user equipments. The principle of the VAM technique is depicted in FIG. 2. The VAM operation/function 100 can be performed as a baseband function after the mapping onto physical channels for Rel'99 and HSDPA (in case of SIMO operation) and after precoding for MIMO. The VAM operation/function can also be implemented in logic in a radio unit such as a Remote Radio Head (RRH). The signals 151, 152 shown at the input of adding operations 150 and 180 are, for example, the resulting channels after MIMO precoding (i.e. MIMO channel #1 21 and MIMO channel #2 22) as explained before and shown in FIG. 1. These signals are mapped (150, 180)) onto Primary and Secondary Common Pilot Channels (P-CPICH 153 and S-CPICH 154) (in case of non-MIMO operation, only one signal 151 will be used and it will be mapped onto a Primary Common Pilot Channel). VAM consists of mapping input signals onto the physical antennas with specific weights for each path. VAM can be seen as a matrix of four weights p1, p2, p3, p4 and two adders 110 applied to two input signals fed by “virtual antennas” 160, 170 corresponding to the physical antennas of the MIMO operation. The VAM weights fulfill totally different objectives than the MIMO precoding weights. These VAM weights are fixed at cell level (typically same VAM weights are used in all cells of a network) objective to fulfill the requirements highlighted above and are applied to all physical channels of the cell (contrary to MIMO weights, which are selected (e.g. every every 2 ms), for each MIMO user equipment and apply to physical channels of the specific MIMO user equipment).
The force of the virtual antenna concept is that the UE behaves as if the signals present at the virtual antennas are the ones actually transmitted, although the physical antennas (120, 130) radiate something different. As stated before, the legacy UE (not supporting MIMO) will only use the virtual antenna 160. Whilst its signal will be transmitted on both physical antennas the UE receiver will act as if transmitted from one (the mapping between virtual and physical antennas is transparent for the user equipment). The configuration received by the legacy user equipment is the same as in a single antenna transmission system, the user equipment is not configured for any form of transmit diversity at RRC level. The MIMO UE will use both virtual antenna 160 and virtual antenna 170 and is unaware of the mapping between the virtual and physical antennas, which is transparent to the MIMO operation.
The four weights from the VAM matrix are differentiated by phases only as equal amplitude is required to achieve power balancing between the two physical antennas 120,130. A first power amplifier 140 and a second power amplifier 190 are configured for amplifying the output signals after the VAM function before they are radiated by the physical antennas 120,130. A pure SIMO mode can be also seen as a particular case of the VAM application, where the same signal from the primary virtual antenna is mapped on the two antennas but with a given phase offset (it is like the second virtual antenna has zero values in the matrix, p3=p4=0). From the legacy user equipment (non-MIMO) point of view the VAM technique is like a single antenna transmission, i.e. the user equipment demodulates the HSDPA signal as if there were no Transmission diversity in the system. Seen from the transmit side for legacy non-MIMO user equipments, VAM amounts to transmitting the same signal (common channel, Rel'99 and HSDPA non-MIMO) on the two transmit antenna ports but with a different phase (different weights p1 and p2).
However, from extensive field testing of VAM functionality (measurements over a large amount of static points which statistically shows the impact of VAM), it is shown that even though the VAM technique has a better performance than previously used techniques such as STTD, it has still a negative impact in HSDPA legacy devices when there is concurrent HSDPA and MIMO traffic. This may be solved with the E-VAM technique.
The Enhanced Virtual Antenna Mapping (E-VAM as disclosed in the European Patent Application EP10382262) is a technique which solves the aforementioned problems by adding to the standard VAM technique the additional functionality whereby an additional phase offset (FIG. 3, 30) (also called adaptive or dynamic phase offset as it is changed dynamically depending in such a way to provide the best performance in the cell) is applied (FIG. 3, 29) to one of the physical paths (1 physical antenna), in order to modify and adapt the transmit polarization according.
In order to select said additional (or dynamic) phase offset, usually a phase scan is made sampling the phase range by given steps of Δθ degrees and measuring user equipments received quality for each phase offset. The phase offset to be applied for the cell may be selected as a function of the measured quality (e.g. cqi) reported by user equipments (FIG. 3, 31) and a given optimisation criterion. Said criterion may be to maximize the throughput of the legacy HSDPA devices especially when MIMO user equipments are active or, in other words to maximize the energy received from the HSPA serving cell by the legacy HSDPA user equipments.
The most used optimization criterion consists of selecting the Phase which maximizes equation
      1    N    ·            ∑              i        =        1            N        ⁢          C      ⁢                          ⁢      Q      ⁢                          ⁢                        I          i                ·                  s          i                    where si is the weight associated to user equipment i, CQIi corresponds to the CQI (channel quality information) reported by user equipment i and N is the number of user equipments considered.
Instead of CQI, other alternative parameters can be used to select the phase as CPICH (Control Pilot Channel) RSCP (Received signal code Power), CPICH Ec/No, NACK info etc.
Said phase scan and phase offset selection may be is made periodically during the duration of the HSDPA session and additionally triggered when there is a new call setup or any other specific event which leads a data user to be in active mode.
E-VAM can only act on a single phase offset (cell level parameter) so it is not possible to adapt the phase offset on a user equipment basis when we have multiple user equipments, the selected phase offset is a compromise to achieve the highest capacity. Usually only quality information reported by HDSPA user equipments is taken into account for the phase selection but it is important to only take into account users which are genuinely active during the application of the phase offset. This may be done, for example, by monitoring for each connected user equipments (user equipments with an HSDPA RAB in cell_DCH state) the volume of data transmitted over a time window which can be the duration of the phase scan period (100-300 ms) or the phase application period (e.g. 1-3 sg). A threshold is then fixed to consider the user equipment as active and susceptible to benefit from E-VAM and, therefore, only these user equipments considered as active are used to calculate the phase offset.
E-VAM is applying this additional phase offset to all user equipments of the cell: MIMO and non-MIMO. However for MIMO user equipments, the E-VAM technique does not have a positive effect (maximizing the throughput). The reason is the following:
The selection of the E-VAM additional offset (scanning, measuring the quality reported by the users . . . ) takes some time, so the E-VAM additional offset are only changed every period of seconds (e.g. 1-3 s). As explained before, the transmissions to MIMO user equipments have pre-coding weights (w1, w2, w3, w4) which are changed by the node B every TTI (e.g. 2 ms), making that the weights applied to the signals transmitted to MIMO user equipments can change every TTI. Hence the E-VAM additional offset effect is somehow superseding by the MIMO precoding weights, because said MIMO weights change so fast compared to the E-VAM offset selection, that the selected phase offset is not any more the one which is optimum for the MIMO user equipments (as their signals parameters changes much faster). In other words, the E-VAM gain is not appreciated by MIMO user equipments. That's why, the MIMO user equipments are not usually taking into account for the E-VAM phase offset selection.
Extensive field testing has shown the following:                Without using E-VAM, MIMO configuration performance is much better than SIMO configuration performance in good radio conditions (40% higher throughput) and in medium radio conditions (10-20% higher throughput). In bad radio conditions, MIMO versus SIMO performance gain ranges typically from 0 to 10%.        However, when E-VAM is used, comparing the performance of MIMO user equipments vs SIMO user equipments, it has been shown that, in good radio conditions the MIMO performance is better (more than 30% higher throughput than SIMO) but, in medium and bad radio conditions, MIMO performance is worse than SIMO (10-15% less throughput with MIMO) due to the E-VAM gain in SIMO user equipments (not appreciable by the MIMO user equipments as explained before).        
This problem does not appear in LTE systems, as in LTE MIMO performance is more robust and always superior to SIMO whatever the radio conditions. This is due to the fact that in LTE technology is embedded instantaneous fallback to transmit diversity which allows a good performance in medium and bad radio conditions. In 3G this is not possible, as the fast fallback is not supported and also because the transmit diversity scheme (STTD) has performance problems (incompatibility with HSDPA UE receiver equalizers).
In other words, MIMO user equipments in 3G has a significant problem of performance versus SIMO user equipments in medium and bad radio conditions (e.g. at cells edges), since MIMO performance is either low or inferior in these radio conditions according to the SIMO configuration used, when using E_VAM technique (which is more spectral efficient).
This makes it very difficult to deploy MIMO or Multi Carrier MIMO functionalities as they would all have this big disadvantage bearing in mind that medium or bad radio conditions performance (e.g. cell edge performance) is very important for an operator.
There is therefore a need in the art for transmission schemes which further improve the performance for MIMO user equipments.